Typically, an optical pickup transport mechanism uses either a lead screw or a rack gear (referred to as a rack from here on). In the case of an optical pickup transport mechanism using a rack, because components including from a pickup motor to an optical pickup are comprised of a gear series and the rack, it is necessary to remove backlash in the optical pickup transport mechanism which is caused by backlash of the gears. Conventionally, as a method of removing backlash, a technology of pushing an optical pickup toward a direction of the inner radius of a disc with a spring is used (for example, refer to patent reference 1). An example of this method will be explained with reference to FIG. 4.
Assuming the sheet of FIG. 4 as the base chassis of a playback unit, a turntable 10 driven by a motor is assembled in this base chassis. Guide members 11 and 12 are arranged around the turntable 10 in such a way as to be parallel to each other, and an optical pickup 2 is placed in such a way as to slide on these guide members 11 and 12 to move in a straight line toward a traveling direction (an arrow A). The optical pickup 2 moves from an inner peripheral position 2a of a disc 20 which is mounted onto the turntable 10 and is rotating to an outer peripheral position 2c of the disc at the time of playback of the disc, and reads recorded data on the disc 20. The optical pickup 2 is driven by delivering and slowing down the rotation of a motor (not shown) for driving the pickup by using a gear series including a pinion 4, and by converting the rotation into straight-line motion by using a rack 6 fixed to a lateral portion of the optical pickup 2. A spring 8 is disposed in order to remove backlash in the optical pickup transport mechanism which is caused by backlash of the gear series including the pinion 4 and the rack 6. The spring 8 has an end 8a fixed to the base chassis and another end 8b fixed to either the optical pickup 2 or the rack 6, and is placed in such a way as to be parallel (including approximately parallel) to the rack 6 and exerts a pushing force in such a way that the rack 6 is always in contact with the gear teeth of the gear series. As a result, backlash is prevented from occurring among the gears.    [Patent reference 1] JP, 2001-297544, A
In the conventional mechanism of transporting the optical pickup, although backlash which is caused by backlash of the rack and the gear series is removed as mentioned above, the elongation of the spring increases from d1 to d2 according to the travelled distance of the optical pickup, as shown in FIG. 4, because the spring is extended in the same direction as that in which the optical pickup is moved. In a case of a disc having a small diameter such as an MD, no problem arises because the travelled distance of the optical pickup is short and the elongation of the spring is therefore small. In contrast, in a case of a disc having a large diameter such as a CD or DVD, the variations in the load of the spring on the optical pickup become remarkable because the elongation of the spring becomes large according to the position to which the optical pickup is moved. As a result, the torque of the pickup motor is affected, and therefore the transporting operation is made unstable and control settings become complicated.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide an optical disc device that makes it possible to reduce the amount of variations in a spring's pushing force for bringing a rack into contact with the gear teeth of a gear series with respect to the travelled distance of an optical pickup.